LTE-Advanced is generally known as a developed form of LTE (Long Term Evolution) that is a radio communication method prescribed by 3GPP (Third Generation Partnership Project).
In LTE-Advanced, the implementation of carrier aggregation is agreed upon. Carrier aggregation is a method in which a single mobile station carries out uplink or downlink communication by simultaneously using a plurality of carriers. Each of the plurality of carriers that is used at the time of carrier aggregation is referred to as a component carrier (CC).
Here, a cell that is connected to the mobile station and that can be used in communication is referred to as a serving cell. Among serving cells, the cell that is initially set when establishing a connection of the mobile station is the primary serving cell (PCell), and a cell that is set after establishing connection is referred to as a secondary serving cell (SCell).
According to Non-Patent Document 1, a mobile station is able to switch the state of each SCell between active state and inactive state. A mobile station does not carry out uplink communication using an SCell in the inactive state, and further, for such an SCell in inactive state, does not measure quality, does not report Channel State Information (CSI) to a base station, and does not monitor Physical Downlink Control Channels (PDCCH).
There are two methods, the following first method and second method, for switching an SCell in the active state to the inactive state.
In the first method, a base station transmits to a mobile station an MAC (Media Access Control) control element for causing deactivation. The mobile station, upon receiving the MAC control element that instructs deactivation of a particular SCell, causes the transition of the state of the SCell to the inactive state.
In the second method, no uplink or downlink communication resources that use PDCCH are allocated to a mobile station by the time of expiration of timers for each SCell in the mobile station. The mobile station similarly causes the state of a particular SCell to transition to the inactive state when there is no allocation of the above-described resources by the expiration of the timer for the particular SCell.
In the method of switching an SCell that is in the inactive state to the active state, a base station transmits a MAC control element that instructs activation. The mobile station, upon receiving the MAC control element that instructs the activation of a particular SCell, causes the transition of the state of the SCell to the active state.
The chief object of providing the above-described second method (deactivation resulting from expiration of timers) other than the above-described first method (transmission of MAC control information that instructs deactivation) is for the purpose of protection when the mobile station is unable to accurately receive the MAC control information that is transmitted in the above-described first method. In this case, the failure to accurately receive the MAC control information that instructs deactivation of a particular SCell results in the possibility that the base station and the mobile station will each have different perceptions such that the mobile station will continue to perceive the SCell as still in the active state whereas the base station will perceive the SCell as being in the inactive state. The above-described second method enables solving this problem, and according to the above-described second method, after the passage of a fixed time interval (the expiration of a timer), the disagreement in perception between the base station and the mobile station relating to the state of the SCell can be eliminated.
However, due to the existence of the above-described second method, a new problem arises in which “in the interval that the base station perceives the active state, the mobile station autonomously transitions to the inactive state, with the result that disagreement occurs between the perceptions of the base station and mobile station,” as will next be explained. The mechanism of the occurrence of this phenomenon is next described more concretely.
Even if a base station uses PDCCH for a particular SCell and instructs the allocation of communication resources, the reception of PDCCH may fail due to the reception quality of the mobile station and communication resources may not be allocated to this SCell. As described hereinabove, when the allocation of communication resources does not succeed by the time of expiration of the timer of a mobile station, the mobile station switches the SCell to the inactive state. At this time, the base station perceives the same SCell as being in the active state, but because the mobile station perceives the inactive state, a disagreement in the perception of the state of the SCell arises.
However, according to Non-Patent Document 2, in Aperiodic CSI transmission, a mobile station is able to transmit a maximum of a 5-serving-cell portion of CSI on one serving cell. Here, CSI includes a channel quality indicator (CQI), a Pre-coding Matrix Indicator (PMI), and a Rank Indicator (RI).
According to Non-Patent Document 3, a unique serving cell index is given to each serving cell within a mobile station. In this case, the serving cell index of a PCell is always “0.”
According to Non-Patent Document 4, the CQI/PMI and RI of each serving cell are each connected in the order of serving cell indices and transmitted on one serving cell. At this time, the CQI/PMI or RI of a serving cell in the inactive state is not connected.
In addition, according to Non-Patent Document 4, when the total number of bits of CQI/PMI that are connected as described hereinabove is 12 or more, an 8-bit Cyclic Redundancy Check (CRC) code is added to enable error detection. On the other hand, when the number of bits is 11 or less, a CRC code is not added and error detection is not possible.